Even the balmy tropics may have once been crushed by colossal masses of ice.
A severe ice age dominated Earth, some 720 to 635 million years ago, well before dinosaurs stalked the land. But it's uncertain if the entire world froze over during the glaciations of this long-lived, extreme episode, dubbed "Snowball Earth." Now, new evidence from Colorado, an area that once was located in the tropics, suggests mighty glaciers sprawled over even Earth's warmest regions.
"This study presents the first physical evidence that Snowball Earth reached the heart of continents at the equator," Liam Courtney-Davies, a geologist at CU Boulder who led the new research, said in a statement. The research was recently published in Proceedings of the National Academy of Sciences, a leading science journal.
A dramatic decrease in greenhouse gases that trap heat on the globe -- notably carbon dioxide -- may have triggered the slide into a frozen orb. Previous research has suggested that even though Earth's average temperature dropped well below freezing, some of the ocean surface and land resisted turning to or getting blanketed in thick, hard ice.
But evidence in the lofty Colorado mountains points to heavy ice in the tropics.
The telltale rocks are called Tava sandstone, which are exposed in the Rocky Mountains. As you might have guessed, sandstone started out as sand on the surface, but is compacted together by different activities, such as grains in rivers becoming buried by layers and layers of silt. In this case, the researchers say the Tava sandstone resembles the type of features found underneath ice sheets, like those in present-day Antarctica.
To see if heavy glaciers may have compacted this once-tropical sandstone (shown below), the researchers dated the mineral veins that crossed through the sandstone. They shot these ancient, rusted minerals with a laser, which discharged uranium, a common radioactive element in Earth's crust. Crucially, uranium naturally decays at a constant rate, providing a dating technique and a good idea of when something formed.
The result? The uranium dating lines up with the period of Snowball Earth, hundreds of millions of years ago.
This is a compelling find, suggesting great glaciers compressed a once tropical Colorado. But this is just one surveyed area. More surveys are needed to truly grasp the extent of this vast ice cover on Earth.
"We want to get the word out so that others try and find these features and help us build a more complete picture of Snowball Earth," Courtney-Davies said.
Eventually, the extreme ice age ended, perhaps stoked by a massive asteroid strike that blew a plethora of heat trapping gasses into the atmosphere, melting the ice and forcing weighty glaciers to retreat. Earth's crust continues to reveal our planet's wild ride over billions of years -- a ride that continues today on Earth's ever-moving surface.